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Acetylene–argon plasmas measured at a biased substrate electrode for diamond-like carbon deposition: I. Mass spectrometry

This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2011 Plasma Sources Sci. Technol. 20 015003 (http://iopscience.iop.org/0963-0252/20/1/015003) View the table of contents for this issue, or go to the journal homepage for more

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PLASMA SOURCES SCIENCE AND TECHNOLOGY

Plasma Sources Sci. Technol. 20 (2011) 015003 (16pp)

doi:10.1088/0963-0252/20/1/015003

Acetylene–argon plasmas measured at a biased substrate electrode for diamond-like carbon deposition: I. Mass spectrometry∗ A Baby, C M O Mahony and P D Maguire Nanotechnology and Integrated Bio-Engineering Centre (NIBEC), University of Ulster, Newtownabbey, BT37 0QB, UK E-mail: [email protected]

Received 25 November 2008, in final form 24 July 2010 Published 7 January 2011 Online at stacks.iop.org/PSST/20/015003 Abstract We report, for the first time, quadrupole mass spectrometry of neutral and positive ionic hydrocarbon species measured at the rf-biased substrate electrode of an inductively coupled plasma for acetylene rich C2 H2 : Ar mixtures under various bias, frequency and pressure conditions. It has been observed that, irrespective of initial gas mixture, the resultant plasma is dominated by argon neutrals and ions. This is attributed to highly efficient conversion of acetylene to C2 H due to the enhanced electron density compared with a standard capacitive plasma where the acetylene (neutral and ion) species remain dominant. This conversion may be crucial to film formation via inert rather than hydrocarbon ion bombardment. In addition, the transient formation of CH4 from acetylene has been discovered using IR absorption spectroscopy with time constants similar to observed pressure variations. Rate coefficients and rates for many of the reaction mechanisms, calculated using measured electron energy distribution functions and species densities, are given. These results have important application in plasma models and growth studies for hydrogenated amorphous or diamond-like carbon film deposition. Film growth under similar plasma conditions is reported in an associated paper along with ion energy distributions for important growth species. (Some figures in this article are in colour only in the electronic version)

expanding thermal plasma [9, 10] and to a limited extent from inductively coupled plasma (ICP) systems [11–13]. These studies focussed mainly on material properties’ dependence on the key input variables, e.g. chamber pressure, power, flow rate, etc. However, future applications, for example in the biomedical arena, will require deposition at higher pressures or onto complex three-dimensional shapes. Hence a much greater understanding of film growth mechanisms which in turn depend on plasma–substrate conditions is necessary. The growth of hydrogen-free DLC (tetragonal amorphous carbon, t-aC) is well understood in terms of carbon ion bombardment as the dominant species and sub-plantation or incorporation of the carbon into the growing film [14–16]. However, hydrogenated DLC (a-C : H) has greater potential application, yet its growth mechanism is highly complex

1. Introduction Diamond-like or amorphous carbons (DLC), particularly hydrogenated amorphous carbon (a-C : H), represent a class of technologically important thin film materials. The ability to vary properties, such as hardness, Young’s modulus, surface roughness, electrical resistance, thermal conductivity, density, refractive index, offers considerable versatility in mechanical, electrical, optical and more recently biomedical applications [1]. These films are routinely deposited from hydrocarbon precursors, particularly C2 H2 , in conventional radio frequency (rf) capacitively coupled plasma (CCP) systems [2–8], ∗

This paper was presented as an invited talk at the 19th European Sectional Conference on Atomic and Molecular Physics of Ionized Gases, Granada, Spain, 15–19 July 2008. See stacks.iop.org/PSST/18/3. 0963-0252/11/015003+16$33.00

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Plasma Sources Sci. Technol. 20 (2011) 015003

A Baby et al

and as yet not understood [17]. A critical impediment is the lack of knowledge of substrate bombardment species, their energies and fluxes, from hydrocarbon-based plasmas, particularly acetylene, which is the preferred precursor gas over methane [4, 16]. There has been some attention given to gas-phase species and reactions in methane containing plasmas [2, 11, 13, 18, 19] and to a much more limited extent to pure acetylene plasmas [20–23] for understanding polymerization reactions, nanoparticle (dust) growth and DLC film formation. Vasile and Smolinsky [23] examined by mass spectrometry the ion chemistry of a pure and mixed acetylene discharges to determine the major reaction paths and highlight the main neutral–neutral and ion–neutral reactions. In their work on dusty plasmas, Deschenaux et al [20] studied mass and energy spectra of dominant ions and neutrals in methane, acetylene and ethylene plasmas. Macek and Cekada [24] used mass–energy spectrometry of C2 H2 /Ar in a triode ion plating apparatus and revealed a high degree of acetylene decomposition. However, the spectra were complicated by the presence of Ti species from an in situ evaporation source. Rangel et al [25] studied the electrical and optical properties of a polymerizing C2 H2 /Ar plasma where OES was used to study mainly H and CH neutral species. Doyle [22] compared the kinetics of a methane rf glow discharge with measured gas species production and depletion rates in pure acetylene discharges. One-dimensional fluid models for pure acetylene plasmas were developed by De Bleecker et al [21] and Herrebout et al [26] in order to understand the basic electron impact, ion–neutral and neutral–neutral reactions underlying the growth of nanosized (or dust) particles. Other models include a zero-dimension kinetic model [27] of a rf C2 H2 (1%)/H2 /Ar-rich plasma to determine the influence of pressure and power in the synthesis of nanodiamond thin films. The existing experimental data and models of C2 H2 -based plasma species have provided a framework for understanding the complex gas-phase reactions; however, their applicability to film growth conditions is much less appropriate. Typical deposition pressures are much lower ( 0 V). Further, in all groups, all observed species except C4 H2 and C4 H4 are present in significant quantities when the plasma is off (Vdc = 0 V). CCP IEDs, since they are measured at the grounded wall, cannot accurately represent the distributions at the driven electrode. Figure 6(a) shows the ratio of count rates normalized to that of C2 H2 at Vdc = 475 V for group I members. The dominant C2 H radical is an important precursor for a-C : H growth, (figure 6(a)). A similar plot for groups II and III, figure 6(b), shows the significant hydrocarbon species to be C2 H4 and C4 H2 . 3.3. ICP neutrals Figure 7 shows examples of the neutral count rates measured for both plasma-off and plasma-on for a range of pressures and electrode bias conditions. For all data the C2 H2 : Ar flow ratio was 2 : 1 and ICP power was 200 W. The upper scan limit was set at 70 amu to maximize analyser operational lifetime. 5

Plasma Sources Sci. Technol. 20 (2011) 015003

A Baby et al

Figure 7. Neutral mass spectra in the ICP C2 H2 : Ar (flow ratio 2 : 1) for various pressures, Vbias and fbias . Data in black show plasma-on. Data in white show plasma-off. Also included is a mass spectrum at base pressure ∼2 × 10−6 Torr.

Figure 8. Chamber pressure and C2 H2 and CH4 concentrations versus time in the ICP: 10 mTorr set pressure, C2 H2 : Ar (flow ratio 2 : 1). The plasma was turned on at t = 20 s and bias (28 V, 8.311 MHz) at 35 s. Both plasma and bias were turned off at 100 s.

time evolution of the pressure and the IR absorption signal for C2 H2 and CH4 species upon plasma switch-on, figure 8 (10 mTorr). The IR absorption measurements provide line integrated concentration values in the bulk plasma 110 mm above the substrate. At 10 mTorr, the C2 H2 signal is seen to fall with an identical time constant τ to that of the pressure (2.5 s). The CH4 signal is barely visible above the background noise; however, at higher pressure, figure 9, its presence is significant. Here the CH4 is seen to rise rapidly ( 2) have only one valence electron to bind to the surface and hence need a dangling bond to stick. Also, subsequent incoming particles cannot simply chemisorb onto these molecules. Closed-shell neutrals have sticking coefficients of almost zero [65]. Unsaturated hydrocarbon radicals (e.g. C2 H or C2 H3 ) have a sticking coefficient of ∼1 and their chemisorption only weakly depends on any surface activation [42] and for C2 H, new dangling bonds can be created on adsorption, if the hybridization changes [17]. For small Cx H radicals, one carbon atom is shielded by the hydrogen and rarely sticks hence their reactivity may be expected to be lower than the equivalent Cx radical where two atoms can stick [66]. Cyclic radicals, e.g. C3 and C3 H, are structurally unstable and also each atom has an available electron for surface binding so that a high sticking coefficient can be expected [66]. The flux of hydrogen species is also important. Atomic hydrogen can create chemisorption sites (dangling bonds) by the abstraction of surface bonded H to form H2 , which desorbs into the plasma, or it can adsorb and passivate dangling bonds in a competing process. Successive ion-induced bond breaking and H-atom passivation within the surface layer can lead to chemical erosion via the formation of weakly bonded Cx Hy species which can diffuse to the surface and desorb [67, 40]. In our ICP system, the spectrometer orifice and vicinity are subjected to ion bombardment with a higher energy and flux compared with the CCP case, where the EQP is positioned at the grounded wall. Thus the sticking efficiency of radicals and the ion/H-atom induced volatile production are likely to differ significantly. Radicals with a low H number, e.g. C2 H, are mainly formed in C2 H2 -based plasmas and often such highly reactive radicals have a gas-phase lifetime much shorter than the diffusion time to chamber walls [68] and estimates of species lifetimes and diffusion rates for our ICP are in agreement. These gas-phase reactions may involve polymerization reactions, particularly the addition of C2 H to the parent molecule, leading to long-chained poly-acetylene molecules. Negative ions are also formed in these plasmas, mainly through dissociative electron attachment, and with successive anion–C2 H2 reactions, gas-phase nanoparticles can grow. Subsequent heterogeneous interactions between radicals and such gas-phase surfaces may be the source of additional volatiles, later detected at the spectrometer. The direct flux of nanoparticles to the spectrometer orifice (and subsequent fragmentation) is only possible for (i) positively charged particles which therefore must have very small diameters or for (ii) very large diameter particles where the force of gravity overcomes the sheath field. In the ICP system, the introduction of C2 H2 into an argon plasma resulted in a reduction in electron density by a factor of >2 and a slight fall in electron temperature, figure 10. The former, along with C2 H2 molecule consumption, figure 8, could be indicative of negative ion formation and particle growth. The concentration of C2 H− anion species is considered a crucial parameter for the initiation of particle growth [70]. The ICP production rate for C2 H− via electron dissociation is estimated to be 1 × 1016 m−3 s−1 , four orders lower than the Ar + direct ionization

rate. Negative ion density is enhanced by trapping within the plasma but estimation of lifetimes is particularly complex. Other important factors related to particle formation in C2 H2 plasmas include an associated increase in electron temperature, which is not observed in our case, and the concentration of source acetylene species [69]. Particle generation has been reported at higher pressures (>75 mTorr) than used here [69, 20]. It is therefore unlikely that significant generation of particles is occurring under our ICP experimental conditions despite the presence of a small number (70) positive ionic species, figure 11. The C2 H2 partial pressure drop at plasma turn-on for mixtures, typically from 6.7 mTorr to 7.6 eV, to lead to an argon dominated plasma irrespective of input gas ratio. The detection of C2 H was inhibited due to high surface loss probability and a significant proportion of the observed C2 H neutrals is due to the species generation within MEA. Depletion of the acetylene precursor was further investigated using infrared spectroscopy, where transient production of CH4 , showing a hitherto unreported C2 H2 –CH4 relationship, was also observed. This transient CH4 signal then decayed to 14

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A Baby et al

Table 2. Rate coefficients k and rates r for selected reactions in the C2 H2 : Ar mixture ICP (I) and CCP (C), f is the ratio of rate coefficient derived from realistic EEDFs to those from the Maxwellian distribution with the same Te . Reaction

Equation

e + Ar → Ar + + 2e

(15)

e + C2 H2 → C2 H2+ + 2e

(3)

e + Ar → Ar m + e

(16)

e + Ar m → Ar + + 2e

(17)

e + C 2 H2 → C 2 H + H + e

(1)

e + C2 H2 → all dissociation

(2)

e + C 2 H6 → C 2 H4 + H 2 + e

(6)

C 2 H + C 2 H2 → C 4 H2 + H

(8)

e + C 2 H2 → C 2 + H 2 + e e + C 2 H4 → C 2 H2 + H 2 + e e + C 2 H4 → C 2 H + H 2 + H + e e + C 2 H6 → C 2 H3 + H 2 + H + e e + C2 H6 → C2 H2 + 2H2 + e e + H2 → H + H + e e + C2 H2 → C + CH2 + e e + C2 H2 → 2CH + e e + CH4 → CH2 + H2 + e Ar + + H2 → ArH+ + H e + C2 H2 → C+ + CH2 + 2e Ar + + C2 H2 → C+ + CH2 + Ar

(10) (10) (11) (11) (10) (21) (12) (14) (22) (18) (19) (20)

I C I C I C I C I C I C I C I C I I I I I I I I I I I I

k (m3 s−1 )

f

r (m−3 s−1 )

Reference

4.5 × 10−16 3.6 × 10−15 1.0 × 10−15 6.0 × 10−15 1.6 × 10−16 5.4 × 10−16 6.0 × 10−14 4.7 × 10−14 1.5 × 10−15 2.8 × 10−15 2.0 × 10−15 4.4 × 10−15 1.4 × 10−14 1.2 × 10−14 5.8 × 10−17

0.66 5.16 0.67 3.82 0.71 2.35 1.02 0.80 0.80 1.53 0.79 1.77 0.99 0.82

[50]

2.8 × 10−16 2.3 × 10−15 1.8 × 10−16 6.5 × 10−17 9.3 × 10−16 1.7 × 10−15 1.0 × 10−16 5.7 × 10−17 5.1 × 10−16 1.7 × 10−15 1.6 × 10−18 1 × 10−19

0.75 0.91 0.77 0.73 0.87 0.79 0.72 0.70 0.81

5.5 × 1019 1.5 × 1020 3.0 × 1016 2.6 × 1020 2.0 × 1019 2.3 × 1019 a 3.6 × 1020 19 a 2.3 × 10 2.8 × 1019 1.3 × 1020 3.8 × 1019 2.0 × 1020 7.3 × 1021 1.5 × 1018 7.2 × 1018 4.2 × 1022 5.4 × 1018 4.7 × 1020 3.6 × 1019 3.3 × 1019 4.8 × 1020 b 1.2 × 1019 17 1.2 × 10 2.2 × 1017 2.8 × 1019 3.9 × 1018 3.0 × 1016 c 6.3 × 1014

0.72

[41] [50] [50] [41] [41] [41] [62] [41] [41] [41] [41] [41] [41] [41] [41] [41] [54] [41] [47]

a The argon metastable ionization rates are calculated for Ar m densities of 3 × 1017 m−3 and 7 × 1017 m−3 for the ICP and CCP, respectively. b Underestimate since cross-sections for all underlying processes are not available. c These reactions only occur in the sheath as they require fast ions.

since this radical is thought to be the dominant sp3 -promoting species during film formation.

almost zero at plasma equilibrium. The rise and fall time constants of C2 H2 depletion were observed to be similar to those of the chamber pressure. Our spectra show heavier masses (>∼50 amu) are not as abundant as those reported in the literature for pure C2 H2 , suggesting argon inhibition of polymerization. However, plasma transparency appeared to be diminished at higher pressures, possible evidence of nanoparticle suspensions; this was not observed at lower pressure. This study was focussed on positive ion and neutral species and negative ions were not investigated. An extensive set of rate coefficients and rates for the suggested electron– neutral and heavy–heavy particle reactions is presented using, where appropriate, realistic EEDFs and measured species densities. These are useful for incorporation in future models. The limitations of mass spectrometry in high deposition flux plasmas, where the instrument orifice is necessarily restricted, have been highlighted. In particular, the orifice can act as a filter for high sticking coefficient species thus limiting the measurement accuracy. These results have important implications for film growth studies and models, which are discussed in [33]. In particular, the argon species dominance in the ICP will result in an alternative ion-bombardment mechanism to current models while the proposed high conversion rate to C2 H is also critical

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